How to get debug output

GUI (Qt5/KF5 instructions)

Press ALT+F2.

Enter 'kdebugsettings' without the quotes and press enter.

Type the name of the kioslave (e.g. ftp, http, ...). If it's not listed, it doesn't use categorized debug output, so either its output is always on, or commented out in the code (in which case it needs to be ported to qCDebug).

Ensure the combobox for the kioslave says "Debug". "All" is misleading since it means "no rule", and the default could be off, not on.

Press OK to close the dialog.

Press ALT+F2, type kdeinit5 and press enter or alternatively log out of KDE and log back in.

This will print additional debug info to the stderr of your kioslave, which typically ends up in ~/.X.err or ~/.xsession-errors

GUI (Qt4/kdelibs4 instructions)

Press ALT+F2.

Enter 'kdebugdialog --fullmode' without the quotes and press enter.

Select the desired number in the "Debug area", e.g. 7103 for http.

In the [Information] box, select "File" as the output.

Enter the desired file name, e.g. /tmp/kio_http.log.

Press OK to close the dialog.

Press ALT+F2, type kdeinit4 and press enter or alternatively log out of KDE and log back in.

This will print additional debug info to the stderr of your kdeinit process, which typically ends up in ~/.X.err or ~/.xsession-errors

Manual (Qt4/kdelibs4 instructions)

It is useful to redirect the debug output of your particular slave to a file
instead of stderr. E.g. I myself use the following lines in
$KDEDIR/share/config/kdebugrc.

This redirects all debug info for areas 7103 and 7113 (as used by kio_http)
to the file /tmp/http.

To get debug information from the SMB slave you can add the following to
kioslaverc:

[SMB]
DebugLevel=100

This will print additional debug info to the stderr of your kdeinit process,
which typically ends up in ~/.X.err or ~/.xsession-errors

How does an io-slave get started?

Your application requests 'klauncher' via DBus for a slave. If 'klauncher' does not have an idle slave ready, it will ask kdeinit to start a new one. kdeinit forks and dlopens the library that contains the io-slave.
Then it calls a function called kdemain() in the library.

Attaching gdb to an io-slave

Due to the above sequence it is rather hard to get an io-slave in your
debugger. But wait there is hope. You can start klauncher in such a way
that slaves for a certain protocol (the first parameter of KIO::SlaveBase() constructor of the slave class) are started in debug mode.

E.g. to start all 'http' slaves in debug mode, you type:

KDE_SLAVE_DEBUG_WAIT=http kdeinit5

This will restart 'kdeinit' and 'klauncher'.

Note:
The string after the equal signal designates the name of a service, not the name of the slave! E.g. if you want to debug the kio_imap4, you must use:

KDE_SLAVE_DEBUG_WAIT=imap kdeinit5

For example, these commands don't work:

KDE_SLAVE_DEBUG_WAIT=imap4 kdeinit5
KDE_SLAVE_DEBUG_WAIT=143 kdeinit5

When your application now requests a http slave, the slave will be started
by kdeinit, but before it calls kdemain() (cq. main()) it will suspend the
slave by sending it a SIGSTOP signal.

In the terminal from which you started kdeinit you will get the following
message:

You can now debug your slave by typing (or pasting) 'gdb kdeinit 16779' in
a terminal. If you don't want to debug a slave you can let it continue by
sending it a SIGCONT by typing 'kill -SIGCONT 16779'.

Be aware that slaves will not be killed while they are suspended.

Once you have started gdb, you can set e.g. breakpoints and then resume the
slave by typing 'continue'. The debugger will return immediate with a message
that a SIGSTOP has been received so you will have to type 'continue' a second
time.